To fix bad posture one of the number one methods is to just stand up straight. That’s right what your mom has been telling you is the best way overall. If you practice sitting and standing straight all day everyday for 30 days; I promise you will be shocked by the results. Your posture will dramatically improve.
To find your normal posture take your shoulders and bring them all the way up and back and then let them hang down. This is your normal posture and a great way to work on shoulder protraction.
Shoulder protraction is caused by a tight chest, tight lats, and a weak upper back (rhomboids). The way how to fix bad posture problem would be working to strengthen the rhomboids twice as much as the chest and lats.
Exercises such as barbell rows and reverse cable flies will greatly help you fix your weak rhomboids. Also spending a lot of time actively and statically stretching the chest and the lats to relieve some of the tightness.
A great chest stretch is done by holding your arms bent at 90 degrees and leaning against a doorway with your chest sticking forward. To stretch the lats hang off of a pull up bar relaxed for at least 20 seconds 4x.
An anterior pelvic tilt is caused by tight hip flexors that pull the pelvis down from the front. Tight lower back muscles pull the pelvis down from the back symbiotically creating a steering wheel turning effect on your pelvis. The glutes are weakened by the position that your hips are in, and are unable to pull the hips into the proper alignment.